The case is the latest conflict in years of legal attacks by the music industry against file-sharing systems like Grokster and Kazaa and their users.

The Recording Industry Association has said that it is unconcerned about the increasing anonymity of file-sharing systems.

That vigilance can include something as simple as having Windows users make sure that Network Neighborhood - a file-sharing system - is turned off.

The available evidence does not show that pornography on file-sharing systems is growing any faster than through other online vehicles.

The music industry took on Napster, said Mr Taylor, because the file-sharing system had no interest in developing the elements needed to turn it into a business.

At Napster, we've developed a payment system and a new file-sharing system, and added everything needed to enable users to share music as they compensate rights holders.

He first uses iTunes to listen to audio samples before obtaining whole albums through BitTorrent, a file-sharing system that is becoming a powerful successor to the Kazaa old guard.

Action was taken in 2006 against more than 10,000 users of file-sharing systems in 18 countries.

Millions of music lovers can now exchange files online every day, using various file-sharing systems.

That is still the case on most such file-sharing systems, said Peter Druschel, a computer scientist at Rice University.